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Selling and trading on ReefCentral.

Yes and no. Depends.
A fish-only may benefit from a filter in the system, but!
Here's a list of useful facts to consider.
1. ONLY live rock and sand can break somewhat-harmful nitrate all the way down to harmless nitrogen gas. Bioballs can't and a filter can't. They take waste down as far as nitrate and stall there. Nitrate builds up and up until there's a cleaning-out of same.
2. Fish can tolerate higher nitrate than corals can. They'd probably rather not tolerate it,...

Run it. Now. To see where you are.
If ever things are going squirrel-ey---test your alk.

...Then test it weekly, during your first half year. Bi-weekly thereafter. This is for fish-onlies AND for reefs both softie and stony. For all tanks. Alkalinity is THE most important test for a reef, once you pass your cycling hurdle. And for a fish-only, alk controls so very much regarding fish health, just consider it an absolute essential. If it's right, you've got a major part of fish-care...